[Contract terminated But I am still paying!]

Dear Magenta,

I really hope someone understands English here, the story is simple:

I terminated my contract in July 2019, I had 6 more months left and agreed to pay the rest of the months on August.

Which I did, on August I received a bill of all of the money that was left, and I was told it is going to be the last one.

I returned my Router box and I did everything correctly, I even have the paper the confirms the cancellation of my account

However I saw a bill of 20 Euros this month, as if I am still a costumer.

I already payed everything I need and I am not a costumer anymore, I need you to stop billing my bank account. (and getting back the money for September would be nice)

Thanks in advance